Simple QKD keyrate computations from tangent-line bounds

• URL: http://arxiv.org/abs/2609.09847v1
• Date: Wed, 09 Sep 2026 07:56:10 GMT
• Title: Simple QKD keyrate computations from tangent-line bounds
• Abstract summary: We show that for basic QKD protocols such as qubit BB84, the keyrates can be easily computed using off-the-shelf solvers.<n>We extend this approach to the six-state protocol, by deriving closed-form expressions for the single-round Rényi entropies.
• Score: 0.0
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: Recent work in quantum key distribution (QKD) has yielded finite-size keyrates based on convex optimisation problems, using the framework of entropy accumulation. Multiple solvers have been developed to address these optimisations in recent years, but have been based on a somewhat elaborate framework using sophisticated algorithms, in order to accommodate a broad range of QKD protocols. In this work, we note that for basic QKD protocols such as qubit BB84, the keyrates can be easily computed using off-the-shelf solvers instead, by obtaining suitable tangent lines to lower bound terms in the objective function. We compute the resulting finite-size keyrates and compare them to previous work. Moreover, we extend this approach to the six-state protocol, by deriving closed-form expressions for the single-round Rényi entropies in the protocol. This should facilitate subsequent study of the six-state protocol via entropy accumulation.

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